In general, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane. The fuel cell includes a membrane electrode assembly (MEA) where an anode and a cathode are provided on both sides of the solid polymer electrolyte membrane. Each of the anode and the cathode includes a catalyst layer (electrode catalyst layer) and a gas diffusion layer (porous carbon). In the fuel cell, the membrane electrode assembly is sandwiched between separators (bipolar plates). A predetermined number of the fuel cells are stacked together to form a fuel cell stack. For example, the fuel cell stack is mounted in a fuel cell electric vehicle as an in-vehicle fuel cell stack.
In certain cases, the membrane electrode assembly has structure where components of the MEA have different sizes, i.e., the surface size (surface area) of one of diffusion layers is smaller than the surface size (surface area) of the solid polymer electrolyte membrane, and the surface size of the other of the gas diffusion layers is the same as the surface size of the solid polymer electrolyte membrane (a stepped-type MEA).
Normally, in the fuel cell stack, a large number of membrane electrode assemblies are stacked together. In order to reduce the cost, there is a demand to produce the membrane electrode assembly at low cost. Therefore, in particular, for the purpose of reducing the amount of expensive material used for the solid polymer electrolyte membrane, and simplify the structure of the solid polymer electrolyte membrane, various proposals have been made.
For example, as shown in FIG. 19, a membrane electrode assembly disclosed in Japanese Laid-Open Patent Publication No. 2007-066766 (hereinafter referred to as conventional technique) includes an electrolyte membrane 1, a cathode catalyst layer 2a provided on one side of the electrolyte membrane 1, an anode catalyst layer 2b provided on the other surface of the electrolyte membrane 1, and gas diffusion layers 3a, 3b provided on both sides of the electrolyte membrane 1.
The surface area of the gas diffusion layer 3b of the anode is equal to the surface area of the electrolyte membrane 1, and larger than the surface area of the gas diffusion layer 3a of the cathode. A gasket structure body 4 is provided in an edge area of the membrane electrode assembly (MEA), and the outer end of the electrolyte membrane 1 adjacent to the gas diffusion layer 3a is joined to the gasket structure body 4 through an adhesive layer 5.